This invention relates to a method of operating mercury cells for the electrolysis of alkali metal chlorides, which comprise virtually horizontal anodes, which are provided on the underside with channel- or groovelike recesses.
It is known that in the electrolysis of alkali metal chlorides the electrode voltage at the gas-producing anode is higher than would correspond to the thermodynamic equilibrium conditions. This phenomenon accounts for part of the overvoltage and is due to the fact that the gas bubbles formed by the electrolysis cover part of the anode surface and block this part of the surface for the flow of electric current. As a result, if the electric current is predetermined, a correspondingly higher electric current will flow through adjacent portions of the anode. The local increase of the electric current results necessarily in a higher voltage in this area. This voltage rise is virtually quantitatively converted into heat and results in a temperature rise of the anode surface. Because the gas cushion on the surface of the anode opposes a rapid exchange of heat, with the electrolyte, there is a relatively poor dissipation of said heat. Eventually, infinitesimal areas of the electrode surface are heated to temperatures exceed 100.degree.C. and are responsible, inter alia, for a corrosion of the anode.
Numerous proposals have been made with the object to reduce this economically undesired overvoltage and to restrict the attack of the electrode surface. For this purpose the anode has been provided with a multiplicity of cylindrical holes or of slots, through which the chlorine gas which has been produced is discharged as rapidly as possible (Opened German Application 1,667,812; 1,792,183; British Patent 1,229,402). In such arrangements it is useful to provide gas flow areas of an order of 15-35%. Larger gas flow areas are avoided because they would involve an excessive effective current density and as a result, an increase of the activation overvoltage. The same purpose is served by numerous proposals for the design of metal anodes, which are made, e.g., of expanded metal, slotted sheet metal elements, or arrangements similar to woven fabrics.
Where the previous proposals are adopted, the gas flows to the surface of the electrolyte along the shortest path which is available and the potential energy possessed by the gas as a result of the hydrostatic pressure of the electrolyte is dissipated at random or, more properly speaking, an unoriented turbulence is produced in the electrolyte. A presence of gas bubbles dispersed in the brine cannot be avoided in the space between the electrodes.
The German Utility Model 7,207,894 describes a further development regarding the design of the flow passages for gas produced by the electrolysis. In that case, the flow passages should be enlarged at least close to and toward one surface of the electrode and may have the shape of a Venturi passages. Whereas that proposal does result in substantial improvements, it cannot entirely prevent gas which leaves a flow passage from entering the suction range of the liquid which flows into the space between the electrodes and from being entrained by said liquid.
In another method of operating cells having a flowing mercury cathode and used for the electrolysis of alkali metal salts, the cover and bottom of the cell are parallel and equally spaced and have an inclination of 2-85.degree. from the horizontal and the cell is filled with electrolyte virtually to the uppermost corner (Printed German Application 1,467,23). It is the object of that arrangement to transmit to the electrolyte the buoyant force exerted on the gas bubbles and thus to effect a circulation in the electrolytic cell so that the gas bubbles are rapidly swept from the anodes. In this connection it is stated that perforated anodes are desirable and that notched or slotted anodes may be used in order to conduct the gas bubbles to the edges of the anode. On the other hand, that proposal has the disadvantage that a large overall height is required and that the filling of the cell to the uppermost corner gives rise to sealing problems at the cover of the cell. These sealing problems will be aggravated since the hydrostatic pressure is increased by a larger inclination of the cell. In that case too an entraining of rising gas bubbles into the space between the electrodes by the suction of the electrolyte cannot be avoided. Finally, the forced circulation of the electrolyte in the cell space above the anodes obstructs the free discharge of the gas which has been produced under the anodes.